1. Field of the Invention
The present invention relates generally to a multiple access communication system, and more particularly, to a method and apparatus for transmitting and receiving data in an Orthogonal Frequency Division Multiplexing (OFDM) system.
2. Description of the Related Art
Having gained recent prominence in high-speed data transmission over wired/wireless channels, OFDM is a particular type of Multi-Carrier Modulation (MCM). In OFDM, a serial symbol sequence is converted to parallel symbol sequences and modulated to mutually orthogonal subcarriers or subchannels, prior to transmission.
The first MCM systems appeared in the late 1950's for military High Frequency (HF) radio communication, and OFDM with overlapping orthogonal subcarriers was initially developed in the 1970's. Since it is difficult to orthogonally modulate between multiple carriers, OFDM has limitations in applications to real systems. However, in 1971, Weinstein, et al's disclosure of an OFDM scheme that applies Discrete Fourier Transform (DFT) to parallel data transmission as an efficient modulation/demodulation process, was a driving force behind the development of OFDM. Also, the introduction of a guard interval and a Cyclic Prefix (CP) as a guard interval further mitigated adverse effects of multi-path propagation and delay spread on systems.
Accordingly, OFDM has been utilized in various fields of digital data communications such as Digital Audio Broadcasting (DAB), digital TV broadcasting, Wireless Local Area Network (WLAN), and Wireless Asynchronous Transfer Mode (WATM). Although hardware complexity was an obstacle to the widespread use of OFDM, recent advances in digital signal processing technology including Fast Fourier Transform (FFT) and Inverse Fast Fourier Transform (IFFT) have enabled OFDM implementation.
OFDM, similar to Frequency Division Multiplexing (FDM), has an advantage of optimum transmission efficiency in high-speed data transmission, in part because it transmits data on sub-carriers, maintaining orthogonality among them. Particularly, efficient frequency use attributed to overlapping frequency spectrums and robustness against frequency selective fading and multi-path fading further increase the transmission efficiency in high-speed data transmission. OFDM reduces the effects of Inter-Symbol Interference (ISI) by use of guard intervals and enables design of a simple equalizer hardware structure. Furthermore, since OFDM is robust against impulsive noise, it is increasingly utilized in communication system configurations.
High-speed, high-quality data services in wireless communications are generally impeded by factors related to the channel environment. The channel environment often changes due to Additive White Gaussian Noise (AWGN), a fading-incurred change in the power of a received signal, shadowing, Doppler effects caused by movement of a Mobile Station (MS) and frequent changes in its velocity, and interference from other users and multi-path signals. Therefore, it is critical to effectively overcome the factors to support high-speed, high-quality data services in wireless communications.
In OFDM, a modulated signal is delivered in the two-dimensional resources of time and frequency. Time resources are distinguished by different OFDM symbols that are mutually orthogonal, and frequency resources are distinguished by different tones that are also mutually orthogonal. A minimum resource unit can be defined with an OFDM symbol on the time axis and a tone on the frequency axis. This is referred to as a “time-frequency bin”. Different time-frequency bins are orthogonal, which prevents signals in the time-frequency bins from interfering with each other in reception.
Under a mobile communication environment, channels change randomly. To avert the resulting problems, most mobile communication systems are designed so as to estimate the channel state of a channel and compensate the channel. This process is called coherent demodulation. For estimation of a random channel state, a signal preset between a transmitter and a receiver should be transmitted. This signal is a pilot signal or a Reference Symbol (RS) signal. The receiver estimates the channel state of the RS signal received from the transmitter and compensates the estimated channel state, for demodulation. As much of the RS signal as sufficient for estimation of a channel change should be transmitted, preferably without being damaged by a data signal. An OFDM system positions the RS signal in time-frequency bins to prevent damage of the RS signal.
FIG. 1 illustrates a conventional RS pattern for two transmit antennas, as defined for a 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) system.
Referring to FIG. 1, one Resource Block (RB) is composed of 12 tones in frequency and 14 OFDM symbols in time. In FIG. 1, a bandwidth having a total of N RBs, first to Nth RBs 121 to 123 (RB 1 to RB N) is shown.
Time-frequency bins 131 marked with “a” represent RSs transmitted through a first antenna, and time-frequency bins 133 marked with “b” represent RSs transmitted through a second antenna. For a single transmit antenna in a Base Station (BS), the time-frequency bins 133 will be used for data transmission. Since the RS signal is preset between the BS and an MS, the MS estimates a channel from the first transmit antenna based on signals received in the time-frequency bins 131 and a channel from the second transmit antenna based on signals received in the time-frequency bins 133.
In the RS pattern illustrated in FIG. 1, some OFDM symbols have RSs and other OFDM symbols are without RSs. Specifically, RSs are defined in 1st, 5th, 8th and 12th OFDM symbols 101, 103, 105 and 107, whereas the other OFDM symbols 111, 113, 115 and 117 are free of RSs. For one transmit antenna, an RS is inserted every six tones, and for the other transmit antenna, RSs are inserted in other RS tones.
FIG. 2 illustrates a conventional RS pattern for four transmit antennas.
Referring to FIG. 2, RSs 131 for a first transmit antenna and RSs 133 for a second transmit antenna are inserted at the same positions as illustrated in FIG. 1. RSs 135 and RSs 136 are additionally defined for third and fourth transmit antennas, respectively. Since the added RSs are positioned in 2nd and 8th OFDM symbols 201 and 203, six OFDM symbols 103, 105, 107, 201 and 203 have RSs among a total of 14 OFDM symbols. The other OFDM symbols 211, 213, 215 and 217 do not have RSs.
To ensure the channel estimation performance of the MS, sufficient power should be allocated to the RS signal. Specifically, when data is transmitted to an MS in a poor channel state, sufficient RS power should be secured because the Signal-to-Noise Ratio (SNR) of the RS signal cannot be improved by retransmission, compared to data for which a certain SNR can be ensured by retransmission. In this context, RS power allocation takes priority over data power allocation. Hence, it may occur that due to allocation of enough power to the RS signal, an available power per data tone is lower in an OFDM symbol with RSs than in an OFDM symbol without RSs.
FIG. 3 illustrates a conventional example of power allocation to data tones in relation to RS power allocation, for a single transmit antenna.
Referring to FIG. 3, reference numeral 301 denotes an OFDM symbol with RSs in an RB, and reference numeral 303 denotes an OFDM symbol without RSs in the RB. The OFDM symbol 301 corresponds to one of the OFDM symbols 101, 103, 105 and 107 illustrated in FIG. 1. The OFDM symbol 301 includes RS tones 311 and data tones 313, while the OFDM symbol 303 has only data tones 315. Power P is allocated to each RS tone 311, higher than power D allocated to each data tone 315 in the OFDM symbol without RSs. In the RB, the condition that a power sum is equal in every OFDM symbol is expressed in Equation (1) asNRS×P+(N−NRS)×D*=N×D  (1)where N denotes the number of tones per OFDM symbol, NRS denotes the number of RS tones in an OFDM symbol with RSs, and D* denotes the power of each data tone in the OFDM symbol with RSs. In FIG. 3, N=12 and NRS=2.
If P>D, DA*<D because N>NRS. That is, as expressed in Equation (2),P−D=(N/NRS−1)×(D−D*)>0  (2)
As described above, the power level of a data tone in the OFDM symbol with RSs is lower than that of a data tone in the OFDM symbol without RSs. Nonetheless, enough power should be allocated first to RSs for reliable communications of every MS in a cell.